I've got a 1.5" layer of XPS foam against a cold concrete wall with
2x4 framing inside that. I temporarily placed a couple pieces of R12 fiberglass batt in the 2x4 wall. When I removed the batts the next day, there was a considerable amount of 'sweat' on the XPS. No moisture was detected on the XPS where it was left uncovered.The area is not directly heated and currently sits around 55 degrees. Exterior temperature is closer to 45 degrees.
Any ideas why condensation would occur between the two materials? It's not obvious to me. The plan was to fully insulate the walls with fiberglass and then cover with a plastic vapour barrier...now I'm not so sure that's a good idea.